444 CHEMICAL ENGINEERING
naterial actually emits only 40 per cent of the intensity of a black body at the same temperature, under the above conditions 60 per cent of the radiation falling ipon it from the walls of the enclosure is reflected, with the net result that the object appears of the same intensity as its surroundings.
However, if the material is removed from the black body and placed in the open air, the reflected intensity is no longer present and the object appears but 40 per cent as bright as a black body at the same temperature. Optical and radiation pyrom-meters are usually calibrated to read correctly when sighted upon a black body. Fortunately many technical processes are carried out under black body conditions. Muffle furnaces, many annealing furnaces, etc., are sufficient approximations to "black bodies" to give practically correct temperature readings with the optical or radiation pyrometer. Some materials in the open are nearly "black," for example the oxide formed on iron and steel ingots, rails, etc.
In general, however, corrections must be applied to the pyrometer readings to obtain the correct temperatures of materials in the open. These corrections are very large in the case of clean molten metals. The presence of an oxide film on the molten metal surface greatly reduces the corrections. For temperature control it is not always necessary to correct the observed readings. As far as the factor of emissivity is concerned the actual pyrometer readings although known to be too low, will always be too low by the same amount from time to time for the same observed temperature, and hence will furnish as good information 'for temperature control and uniformity as could the true temperatures.
Optical Pyrometer Temperature Scale.—The temperature scale for the optical pyrometer is based upon Wien's law for the distribution, in the spectrum, of the energy of a black body. This law may be stated by the following equation where X denotes the wave length in microns, c2 a constant = 14,350, $ the absolute temperature of the black body, J\ the intensity at the wave length X (i.e. at a particular color such as red), and ci a constant, the value of which is of no moment in pyrometry since, as will be seen, it disappears from the actual working equation
Jx = ciX-Be^ W
for a black body. The intensity of radiation Jx1, of wave length X, from a non-black body of temperature & and emissivity E\ is given by equation
Jx1 = dExX-si™* = ciX-V^ (2)
for a non-black body. In the third term of (2) we define S\ as the apparent temperature in degrees absolute of the non-black body. This is the temperature measured by the optical pyrometer and is less than the true temperature & for all materials except black bodies, when it becomes equivalent to $. From (2) we have:
i- _ JL = X log #* __ X log ffx ,^
4 S\ 0.4343c2~" 6232 w
Thus knowing X and E\, it is always possible to obtain the true temperature & from the observed temperature S\.
An optical pyrometer is simply a photometer using monochromatic light (usually , red) in which the intensity of radiation from either a standard or a constant source (electric lamp, oil flame, etc.) is compared with that from the object of which the temperature is desired. Frequently the two intensities are made equal by adjusting various types of absorbing devices (absorption glasses, iris diaphragm, etc.) interposed either on the furnace side or the standard-lamp side of the pyrometer, depending upon